Serious question, a bit of historiography:

Must this question have an answer? Do these things have a "Why"? And by answering it, do we not run the risk of constructing a narrative that fits the facts?

My university has been running a project for a few years now, led by Professor Danny Miller, entitled The Global Denim Project and its aim is understand the phenomenon of denim, its enduring presence in global fashion and how different cultures interact with the fabric on a day-to-day basis. It's a really cool project, so check it out! It's got links to other projects related to denim as well.

For those with access to a university network, here is the original paper that inspired the project. For those who cannot see more than the abstract, the project page linked above provides a good synopsis.

It accompanies the book he wrote on the subject, Blue Jeans: The Art of the Ordinary if anyone is interested in that, too.

I'm sorry that I cannot directly answer your question, I feel it is perhaps a lot more multi-faceted that we think, but I hope that the links above can point you in the right direct and may be of interest!

Edit: I realise that the links I have provided, and the approaches discussed in them, are anthropological rather than historical so not directly helpful for OP. That said, the question itself relies more on anthropology than history. There isn't a singular narrative or specific cause as to why we see denim in every country throughout the world and have done for decades. It's the result of numerous cultures interacting over time.

Tim Gunn offered a pretty good history on the Jeans in his book "The Fashion Bible." Granted the book was written more as a populist view on fashion, but I found it a good overview.

To summarise some of his points, he noted that jeans were not at all popular throughout the years. In fact, it was traditionally treated as a working man's clothes. For this reason, the cut and material of jeans were uncomfortable and not generally consumer friendly. For this reason also, there was very little traction for it in consumer markets.

That was until the movie "Rebel Without a Cause" popularised the item in the 1950s (and cemented James Dean as a sex symbol of his era). This era led to many changes that would popularise and modernise jeans. While the denim weave was still used, the material and thread changed. In addition, the cut changed from the baggy workman's pants to the ones we see today.

At a glance, the 1950s was very influential in defining modern populist fashion. It was also the year that popularised the T-Shirt...